Cover glass for mobile terminals, manufacturing method of the same and mobile terminal device

ABSTRACT

To provide cover glass for mobile terminals exhibiting high strength in a thin plate thickness state to enable reductions in thickness of apparatuses when inserted in the apparatuses, cover glass ( 1 ) for a mobile terminal of the invention is cover glass ( 1 ) that is obtained by forming a resist pattern on main surfaces of a plate-shaped glass substrate, then etching the glass substrate with an etchant using the resist pattern as a mask, and thereby cutting the glass substrate into a desired shape and that protects a display screen of the mobile terminal, where an edge face of the cover glass ( 1 ) is formed of a molten glass surface, and as surface roughness of the edge face, arithmetic mean roughness Ra is 10 nm or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. National Stage of InternationalApplication No. PCT/JP2008/072863, filed Dec. 16, 2008. This applicationclaims the benefit of Japanese Patent Application No. JP 2007-325542,filed Dec. 18, 2007. The disclosures of the above applications areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to cover glass used in protecting adisplay screen of a mobile terminal device such as, for example, acellular phone and PDA (Personal Digital Assistant), manufacturingmethod of the cover glass and mobile terminal device.

BACKGROUND ART

In a mobile terminal device such as a cellular phone and PDA, to preventa shock and external force from being applied to the display, aprotective plate made of plastic, for example, protective plate made ofacrylic resin with high transparency is disposed at a predetermineddistance outside the display (for example, Patent Document 1).

However, since the protective plate made of acrylic resin tends to bendby external force, it is necessary to set the distance between theprotective plate and display at a large extent to which the bending canbe absorbed. Further, it is required to increase the thickness toprovide the protective plate made of acrylic resin with strength to someextent. Therefore, it has become difficult to actualize thinner mobileterminal devices.

Then, to actualize thinner mobile terminal devices, proposed is aprotective plate using chemically strengthened glass which suppressbending and is a thin plate while having strength (for example, PatentDocument 2). Patent Document 2 describes cover glass for mobileterminals obtained by cutting plate glass of a specific glasscomposition into a predetermined shape, chamfering the edge face,performing mirror polishing processing on both surfaces, and thenforming a compressive stress layer in the surface by chemicalstrengthening, thereby suppressing its bending and reducing its tendencyto break, and a manufacturing method of the cover glass.

-   Patent Document 1: Japanese Unexamined Patent Publication No.    2004-299199-   Patent Document 2: Japanese Unexamined Patent Publication No.    2007-99557

DISCLOSURE OF INVENTION

Meanwhile, in recent years, in mobile terminal devices such as cellularphones and PDA, competition among apparatus manufacturers hasintensified, and required are expensive-looking high-fashion mobileterminal devices, as well as sliming down and high functionality ofmobile terminal devices. High-fashion is also required for cover glass,and cover glass is requested in complicated shapes as a shape of theexternal form of the cover glass, starting conventional simple rectangleshapes, such as shapes (for example, shapes with the negative curvaturesuch that some side constituting the cover glass is largely dentedinward) in accordance with the shape of a display screen of theapparatus, and other shapes such that a hole is formed on the mainsurface of the cover glass.

However, in the conventional processing method as described in PatentDocument 2, surface roughness is coarse in the cover glass edge face,micro-cracks ranging from about dozens to hundreds of micrometers existon the chamfering-processed surface of the cover glass edge face, and aproblem thereby arises that it is not possible to obtain mechanicalstrength required for the cover glass for mobile terminals. Further, inthe conventional processing method as described in Patent Document 2,cover glass in complicated shapes as described above is not obtained, orif such cover glass is obtained, the mechanical strength is extremelylow, the processing cost is high, and the cover glass is not put topractical use under present circumstances.

The present invention was made in view of the problems, and it is anobject of the invention to provide cover glass satisfying highmechanical strength required for cover glass for a mobile terminal evenin a complicated shape, manufacturing method of the cover glass and amobile terminal device with a display screen having high mechanicalstrength.

Cover glass for a mobile terminal of the invention is cover glass thatis obtained by cutting a plate-shaped glass substrate into a desiredshape by etching and that protects a display screen of the mobileterminal, and is characterized in that an edge face of the cover glassis formed of a molten glass surface, and that as surface roughness ofthe edge face, arithmetic mean roughness Ra is 10 nm or less.

The cover glass for a mobile terminal according to this configuration isobtained by cutting a plate-shaped glass substrate into a desired shapeby etching without undergoing mechanical processing, thereby has highsmoothness of the order of nanometers such that surface roughness of theedge face of the cover glass is 10 nm or less, as compared with coverglass undergoing external form formation by mechanical processing, andthus has a surface state with extremely high smoothness further withoutmicro-cracks that are certainly formed in performing external formformation by mechanical processing. Therefore, even when the shape ofthe external form of the cover glass for a mobile terminal is acomplicated shape, it is possible to satisfy high mechanical strengthrequired for the cover glass for a mobile terminal.

In the cover glass for a mobile terminal of the invention, the mainsurface of the cover glass is formed of a molten glass surface formed bya down-draw method, and as surface roughness of the main surface,arithmetic mean roughness Ra is preferably 0.5 nm or less. According tothis configuration, the cover glass becomes further excellent inmechanical strength.

In the cover glass for a mobile terminal of the invention, the desiredshape is preferably a shape including a portion having the negativecurvature in part of a contour constituting the cover glass.

In the cover glass for a mobile terminal of the invention, a platethickness of the cover glass is preferably 0.5 mm or less.

In the cover glass for a mobile terminal of the invention, the coverglass is preferably aluminosilicate glass containing at least oneselected from the group consisting of SiO₂, Al₂O₃, Li₂O and Na₂O.According to this configuration, it is made possible to form theplate-shaped glass substrate by a down-draw method (fusion method), andthe main surface of the glass substrate can be a molten glass surfacehaving extremely high smoothness of the order of nanometers withoutflaws. Accordingly, the need for performing mirror polishing processingon the main surface is eliminated in fabricating the cover glass, andthe cover glass without micro-cracks even in the main surface isobtained and becomes the cover glass excellent in mechanical strength.Further, since chemically strengthening by ion exchange is madepossible, it is possible to further enhance the mechanical strength.

In the cover glass for a mobile terminal of the invention, thealuminosilicate glass preferably contains 62 percent to 75 percent byweight of SiO₂, 5 percent to 15 percent by weight of Al₂O₃, 4 percent to10 percent by weight of Li₂O, 4 percent to 12 percent by weight of Na₂O,and 5.5 percent to 15 percent by weight of ZrO₂. According to thisconfiguration, stable glass manufacturing is made possible, and obtainedis glass excellent in mechanical strength. Further, the cover glass fora mobile terminal is used in significantly severe environments such thatthe cover glass comes into contact with human skin, water, rain, etc.and even in such environments, is capable of having sufficient chemicaldurability.

In the cover glass for a mobile terminal of the invention, the coverglass is preferably glass that is chemically strengthened by ionexchange treatment. Further, the cover glass preferably has compressivestress layers in the main surface and the edge face. According to thisconfiguration, the surfaces of cover glass are chemically strengthened,the compressive stress layers are further formed in the surfaces (mainsurfaces and the edge face), and it is thereby possible to furtherincrease the mechanical strength.

In the cover glass for a mobile terminal of the invention, the edge faceof the cover glass preferably has a protruding center portion, andinclined surfaces inclined respectively toward both main surface sidesfrom the center portion. According to this configuration, in insertingthe cover glass in a frame or the like of a mobile terminal device, itis possible to insert the glass with ease without galling and/orchipping occurring.

A method of manufacturing cover glass for a mobile terminal of theinvention is a method of manufacturing cover glass for a mobile terminalfor protecting a display screen of the mobile terminal, and ischaracterized by forming a resist pattern on main surfaces of aplate-shaped glass substrate, then etching the glass substrate with anetchant of a mixed acid aqueous solution containing hydrofluoric acidand at least one kind of acid among sulfuric acid, nitric acid,hydrochloric acid, and hydrofluorosilicic acid using the resist patternas a mask, and thereby cutting the glass substrate into a desired shape.

According to this method, in cutting the plate-shaped glass substrateinto a desired shape by etching without performing mechanicalprocessing, since a mixed acid aqueous solution containing hydrofluoricacid and at least one kind of acid among sulfuric acid, nitric acid,hydrochloric acid, and hydrofluorosilicic acid is used as an etchant,the cover glass having an extremely high surface state is obtained suchthat surface roughness of an edge face of the cover glass cut into thedesired shape has high smoothness of the order of nanometers, and thatmicro-cracks do not exist which are certainly formed in forming theexternal form by mechanical processing. Further, since it is possible toadopt photolithography in forming the resist pattern, dimensionalaccuracy of the cut cover glass is also excellent. Accordingly, evenwhen the shape of the external form of the cover glass for a mobileterminal is a complicated shape, the cover glass with good dimensionalaccuracy is obtained, and it is possible to obtain high mechanicalstrength required for the cover glass for a mobile terminal.

In the method of manufacturing cover glass for a mobile terminal of theinvention, the plate-shaped glass substrate is preferably formed by adown-draw method. This is because both main surfaces of the plate-shapedglass substrate formed by the down-draw method have surfaces formed byhot forming, and thereby have extremely high smoothness of the order ofnanometers, and surface states without micro-cracks. Further, since itis possible to perform etching uniformly from the both main surfaces inetching the glass substrate from the both main surfaces using resistpatterns formed on the both main surfaces of the glass substrate as amask, dimensional accuracy is good, and the shape in cross section ofthe edge face of the cover glass is excellent, thus being preferable.

In the method of manufacturing cover glass for a mobile terminal of theinvention, the desired shape is preferably a shape including a portionhaving the negative curvature in part of a contour constituting thecover glass.

In the method of manufacturing cover glass for a mobile terminal of theinvention, the cover glass is preferably aluminosilicate glasscontaining at least one selected from the group consisting of SiO₂,Al₂O₃, Li₂O and Na₂O. According to this method, since it is madepossible to form the plate-shaped glass substrate by a down-draw method,the main surface of the glass substrate does not have flaws, and obtainsa surface state having extremely high smoothness of the order ofnanometers. Accordingly, the need for performing mirror polishingprocessing on the main surface is eliminated in fabricating the coverglass, and the cover glass without micro-cracks even in the main surfaceis obtained and becomes the cover glass excellent in mechanicalstrength. Further, since chemical strengthening by ion exchange is madepossible, it is possible to further enhance the mechanical strength.

In the method of manufacturing cover glass for a mobile terminal of theinvention, after cutting into the desired shape, it is preferable toperform chemically strengthening on the cut glass substrate by ionexchange treatment. According to this method, the compressive stresslayers are formed in the entire surfaces (main surfaces and the edgeface) constituting the cover glass, and it is thereby possible tofurther increase the mechanical strength.

A mobile terminal device of the invention is characterized by having anapparatus body having a display screen, and the above-mentioned coverglass for a mobile terminal provided above the display screen. Accordingto this configuration, it is possible to provide a mobile terminaldevice with the screen display having high mechanical strength.

In the cover glass for a mobile terminal of the invention, even when theshape is complicated, it is possible to obtain high mechanical strengthrequired for the cover glass for a mobile terminal. Further, in themobile terminal device of the invention, even when the cover glassprotecting the display screen has a complicated shape, it is possible toobtain the mobile terminal device with the display screen having highmechanical strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing part of a mobile terminalequipped with cover glass for mobile terminals according to anEmbodiment of the invention;

FIG. 2( a) is a diagram to explain the external form of the cover glassas shown in FIG. 1; FIG. 2( b) is a diagram to explain the negativecurvature; FIG. 2( c) is a diagram to explain the negative curvature andpositive curvature;

FIG. 3 is a diagram showing a shape of an edge face of the cover glassas shown in FIG. 2;

FIGS. 4( a) to 4(c) are diagrams to explain shape processing by etchingof a glass substrate;

FIG. 5 is a diagram to explain a shape of cover glass of Example 3; and

FIGS. 6( a) to 6(c) are diagrams to explain a manufacturing process toprepare the cover glass as shown in FIG. 2( a) by a manufacturing methodof a Comparative Example.

BEST MODE FOR CARRYING OUT THE INVENTION

An Embodiment of the invention will specifically be described below withreference to accompanying drawings.

FIG. 1 is a cross-sectional view showing part of a mobile terminaldevice equipped with cover glass for mobile terminals according to anEmbodiment of the invention. In the mobile terminal device as shown inFIG. 1, cover glass 1 is disposed at a distance D above a liquid crystaldisplay panel 2. The liquid crystal display panel 2 is configured suchthat a pair of glass substrates 21 and 22 sandwich a liquid crystallayer 23. In addition, in FIG. 1, the other members are omitted whichare generally used in a liquid crystal display panel.

The cover glass in this Embodiment of the invention is obtained byforming a resist pattern on a main surface of a plate-shaped glasssubstrate, then etching the glass substrate with an etchant using theresist pattern as a mask, and thereby cutting the glass substrate into adesired shape, while an edge face 14 of the cover glass 1 as shown inFIG. 3 is formed of a molten glass surface, and surface roughness(arithmetic mean roughness Ra) in the edge face 14 is 10 nm or less.Thus, in the cover glass according to the invention, since the externalform is formed by etching, the edge face 14 formed by etching hasextremely high smoothness, is formed of the molten glass surface, andtherefore, is in a state without micro-cracks that certainly exist on anedge face formed by mechanical processing. In the cover glass with sucha configuration, even when the shape of the external form of the coverglass for mobile terminals is a complicated shape, it is possible toprocess the external form into a desired shape with ease, and it ispossible obtain high mechanical strength required for cover glass formobile terminals.

Further, for example, even when the plate thickness is 0.5 mm or lessand thus thin, it is possible to maintain high mechanical strength. Whensuch cover glass with a thin thickness is inserted in an apparatus,since the cover glass is hard to bend by external force due to the highmechanical strength, it is possible to reduce the distance between thecover glass and display. As a result, it is possible to intend to thinthe apparatus.

In addition, as an etching method to etch the glass substrate, either ofwet etching and dry etching is available. From the viewpoint of reducingthe processing cost, wet etching is preferable. Any etchant capable ofetching a glass substrate can be used as an etchant used in wet etching.For example, it is possible to use an acidic solution containinghydrofluoric acid as the main ingredient, mixed acid containinghydrofluoric acid and at least one kind of acid among sulfuric acid,nitric acid, hydrochloric acid, and hydrofluorosilicic acid, etc.Further, as an etchant used in dry etching, any etchant capable ofetching a glass substrate can be used, and for example, it is possibleto use fluorine gas.

Moreover, the cover glass 1 of the invention can be formed in a shapeincluding a portion having the negative curvature in part of a contourconstituting the cover glass as shown in FIG. 2( a). Herein, thepositive curvature and negative curvature are defined. It is assumedthat the contour constituting the cover glass is represented by curve C1and curve C2 as shown in FIG. 2( b), and that the cover glass isrepresented by area A. At this point, when the contour constituting thecover glass is traced while always seeing the inside of the area A onthe left side, a portion of the contour turning to the left as proceedsis defined as the positive curvature, another portion of the contourturning to the right as proceeds is defined as the negative curvature,and the curvature of a straight portion that turns to neither the leftnor right is defined as “0”. Accordingly, the curve C1 as shown in FIG.2( b) is a curve having the positive curvature, and the curve C2 asshown in FIG. 2( b) is a curve having the negative curvature. Further,also when the contour constituting the cover glass is complicated andbecomes a shape as shown in FIG. 2( c), according to the aforementioneddefinition, a segment C+ is the positive curvature, a segment C− is thenegative curvature, and the curvature of a segment C0 is “0”.

When the negative curvature is defined as described above, the portionhaving the negative curvature in the cover glass 1 as shown in FIG. 2(a) means corner portions 121 of a hole portion 12, corner portions 111of a concave portion 11, etc. which are formed within the main surfaceof the cover glass 1.

Such a shape including portions having the negative curvature is a shapehard to process by mechanical processing that is the external formprocessing in the ordinary manufacturing process of the glass substrate.It is possible to actualize such a shape with ease by usingphotolithography and etching as described later. In addition, the cornerportion 121 of the hole portion 12 and the corner portion 111 of theconcave portion 11 described herein do not include portions caused bychipping of the glass or surface roughness and surface swell comprisedof minute convex portion and/or concave portion formed on the coverglass surface. In other words, such portions do not include a convexportion caused by chipping of the glass in the portion having thepositive curvature, concave portion in surface roughness or surfaceswell, or the like.

The cover glass 1 can be fabricated using a plate-shaped glass substrate(sheet glass) formed by a down-draw method. Among glass enabling glassplate formation by a down-draw method is aluminosilicate glasscontaining SiO₂, Al₂O₃, Li₂O and/or Na₂O. In particular, thealuminosilicate glass preferably contains 62 percent to 75 percent byweight of SiO₂, 5 percent to 15 percent by weight of Al₂O₃, 4 percent to10 percent by weight of Li₂O, 4 percent to 12 percent by weight of Na₂O,and 5.5 percent to 15 percent by weight of ZrO₂. Further, such acomposition is preferable that the ratio by weight of Na₂O/ZrO₂ rangesfrom 0.5 to 2.0, and that the ratio by weight of Al₂O₃/ZrO₂ ranges from0.4 to 2.5.

SiO₂ is the prime component forming a glass skeleton. The cover glassfor mobile terminal devices, particularly, for cellular phones is usedin significantly severe environments such that the cover glass comesinto contact with human skin, water, rain, etc. and even in suchenvironments, is required to exert sufficient chemical durability. Withconsideration given to the chemical durability and melting temperature,the content of SiO₂ preferably ranges from 62 percent by weight to 75percent by weight.

Al₂O₃ is contained to enhance ion exchange performance of the glasssurface. Further, the cover glass for mobile terminals needs to haveviewability i.e. transparency. With consideration given to chemicaldurability and transparent durability, the content of Al₂O₃ preferablyranges from 5 percent by weight to 15 percent by weight.

Li₂O is ion-exchanged with Na ion mainly in an ion exchange treatmentbath in the glass surface portion, and is thus an essential component inchemically strengthening the glass. With consideration given to ionexchange performance, transparent resistance and chemical durability,the content of LiO₂ preferably ranges from 4 percent by weight to 10percent by weight.

Na₂O is ion-exchanged with K ion in the ion exchange treatment bath inthe glass surface portion, and is thus an essential component inchemically strengthening the cover glass. Further, the mobile terminaldevice is used under circumstances where shock and/or external force isapplied to the display screen by dropping the apparatus, pressing thedisplay screen repeatedly, or by opening and closing in the case ofopen/close type mobile terminal devices, and even in such useenvironments, is required to need sufficient mechanical strength. Withconsideration given to the mechanical strength, transparent resistanceand chemical durability, the content of Na₂O preferably ranges from 4percent by weight to 12 percent by weight.

ZrO₂ has the effect of increasing the mechanical strength. Withconsideration given to chemical durability and stable manufacturing ofhomogenous glass, the content of ZrO₂ preferably ranges from 5.5 percentby weight to 15 percent by weight.

Further, in the aforementioned aluminosilicate glass, by performingchemical strengthening by ion exchange treatment, and thereby forming acompressive stress layer in the glass surface, it is possible to furtherincrease the mechanical strength. In addition, as a substitute for thealuminosilicate glass, other multi-component glass may be used. Further,when required transparency is guaranteed as the cover glass for mobileterminals, crystallized glass may be used.

In the invention, the effect is particularly exhibited when thethickness of the cover glass 1 is 0.5 mm or less. Further, as glassconstituting the cover glass 1, it is preferable to use the glasschemically strengthened by ion exchange treatment as described above.The chemically strengthened glass is glass strengthened by replacingalkali metal ions constituting the glass with alkali metal ions largerin size than the alkali metal ions constituting the glass. A compressivestress layer is formed in the surface of thus chemically strengthenedglass.

Described next is a method of manufacturing cover glass for mobileterminals of the invention.

In an external form processing step in manufacturing of the cover glass,the external form processing is performed by performing photolithographyand etching on a glass substrate. In an etching step, as shown in FIG.4( a), both main surfaces of the glass substrate 1 are coated withresist materials 31. Next, the resist materials are exposed through aphoto mask having a pattern with a desired external form shape (forexample, external form including portions having the negativecurvature). Then, as shown in FIG. 4( b), the exposed resist materialsare developed, a resist pattern is formed (an opening portion 31 a isformed) in an area except areas to be etched of the glass substrate, andthe areas to be etched of the glass substrate are etched. At this point,in the case of using a wet etchant as an etchant, as shown in FIG. 4(b), the glass is isotropically etched, and by this means, the edge face14 becomes the shape as shown in FIG. 4( c). In other words, in the edgeface 14, a center portion 14 a protrudes outward the most, and inclinedsurfaces 14 b, 14 b are formed which are gently curved respectivelytoward both main surface 13 sides from the center portion 14 a. Inaddition, it is preferable that the boundary between the inclinedsurface 14 b and the main surface 13 and the boundary (center portion 14a) between the inclined surfaces 14 b are in a rounder shape with aradius of several tens of micrometers. By forming such an edge-faceshape, in inserting the cover glass in a frame, etc. of a mobileterminal device, it is possible to perform insertion with ease withoutgalling and/or chipping occurring.

As a resist material used in the etching step, any material is availablewhich has resistance to the etchant used in etching the glass using aresist pattern as a mask. The glass is normally etched by wet etching ofan aqueous solution containing hydrofluoric acid, or dry etching offluorine gas, and therefore, for example, it is possible to use resistmaterials excellent in resistance to hydrofluoric acid.

As an etchant used in the etching step, it is possible to use a mixedacid containing hydrofluoric acid and at least one kind of acid amongsulfuric acid, nitric acid, hydrochloric acid and hydrofluorosilicicacid, etc. By using the aforementioned mixed acid aqueous solution as anetchant, the cover glass having an extremely high surface state isobtained such that the edge face of the cover glass cut into the desiredshape has surface roughness of 10 nm or less and thus has highsmoothness of the order of nanometers, and that micro-cracks do notexist which are certainly formed in forming the external form bymechanical processing. Further, since photolithography is adopted informing the external form, dimensional accuracy of the cut cover glassis also good. Accordingly, even when the shape of the external form ofthe cover glass for mobile terminals is a complicated shape, the coverglass with good dimensional accuracy is obtained, and it is possible toobtain high mechanical strength required for the cover glass for mobileterminals. Further, by this external form processing by photolithographyand etching, it is also possible to enhance productivity and reduce theprocessing cost. Furthermore, as a remover solution to remove the resistmaterial from the glass substrate, it is preferable to use an alkalisolution of KOH, NaOH, etc. In addition, kinds of the resist material,etchant and remover solution are capable of being selected asappropriate corresponding to the material of the glass substrate that isa material to be etched.

Further, since the processing for external form is performed by etchingstep, it is possible to form the concave portion 11 and hole portion 12including portions of the negative curvature in the cover glass withease. Furthermore, similarly, since the etching step is used, byadjusting the mask pattern, it is possible to add a design (variousshapes of the external form) such as the logo to the cover glass. Bythis means, it is possible to easily actualize a complicated designwhich will be required for the cover glass in the future but cannot beimplemented by mechanical processing.

Furthermore, as the plate-shaped glass substrate, it is possible to useglass substrates that are directly formed in the shape of a sheet frommolten glass, or glass substrates obtained by cutting a glass materialformed in some thickness into a predetermined thickness, polishing themain surface, and finishing in a predetermined thickness. It ispreferable to use glass substrates that are directly formed in the shapeof a sheet from molten glass. This is because the main surface of theglass substrate that is directly formed in the shape of a sheet frommolten glass is a surface formed by hot forming has extremely highsmoothness, and further has a surface state without micro-cracks.Methods for directly forming the molten glass in the shape of a sheetinclude a down-draw method and float method. The down-draw method ispreferable among the methods. In addition to the above-mentioned effectsof high smoothness, etc. in the case of performing the external formprocessing by etching step, since it is possible to perform etchinguniformly from both main surfaces in etching the both main surfaces ofthe glass substrate using resist patterns formed on the both mainsurfaces of the glass substrate as a mask, dimensional accuracy is good,and the shape in cross section of the edge face of the cover glass isexcellent, thus being preferable.

Moreover, for the same reason as described above, the glass in themethod of manufacturing cover glass of the invention is preferablyaluminosilicate glass containing SiO₂, Al₂O₃, Li₂O and/or Na₂O.

Further, by performing chemical strengthening by ion exchange treatmenton the cover glass after the step of the external form processing byetching, a compressive stress layer is formed in the surface of thecover glass, and it is thereby possible to further enhance mechanicalstrength of the cover glass. In the case of using the above-mentionedplate-shaped glass substrate obtained by the method such as thedown-draw method of directly forming molten glass in the shape of asheet, since both main surfaces of the glass substrate are surfacesformed by hot forming, and thereby have surface states with extremelyhigh smoothness without micro-cracks that are surely formed when theexternal from is formed by mechanical processing, the compressive stresslayer formed by chemical strengthening is required to be 5 μm or more.The thickness of the compressive stress layer is preferably 50 μm ormore, and more preferably, 100 μm. Since the cover glass for protectinga display screen of a mobile terminal is provided to cover the displayscreen, flaws are tend to occur in handling, and further, inconsideration of the respect that shock and/or external force is appliedby pressing the display screen repeatedly, or by opening and closing inthe case of open/close type mobile terminal devices, the compressivestress layer is desired to be formed deeply.

Furthermore, the cover glass 1 fabricated by the above-mentionedmanufacturing method of cover glass has the main surface 13 and the edgeface 14 as shown in FIG. 3, and the cross-sectional shape of the edgeface 14 is almost the same in the entire edge face 14 in the externalform as shown in FIG. 2. In the cover glass according to the invention,since the external form is formed by etching as described above, all ofthe obtained edge face 14 of the external form is formed under the sameconditions. Therefore, in the case of forming the external form by wetetching, since the glass is etched isotropically, the edge face 14 is inthe shape as shown in FIG. 3 over the whole of the external form.

Described next are Examples performed to clarify the effects of theinvention.

Descriptions are given below using cover glass for a cellular phone toprotect a display screen of the cellular phone as an example.

EXAMPLE 1

First, aluminosilicate glass containing 63.5 percent by weight of SiO₂,8.2 percent by weight of Al₂O₃, 8.0 percent by weight of Li₂O, 10.4percent by weight of Na₂O and 11.9 percent by weight of ZrO₂ was formedinto plate-shaped glass substrates (sheet-shaped glass) with a sheetthickness of 0.5 mm by a down-draw method. Surface roughness (arithmeticmeans roughness Ra) of the main surface of the sheet-shaped glass formedby the down-draw method was measured using an atomic force microscope,and was 0.2 nm.

Next, both main surfaces of the sheet-shaped glass were coated withnegative type hydrofluoric-acid resistant resist in a thickness of 30μm, and the hydrofluoric-acid resistant resist underwent bakingprocessing at 150° C. for 30 minutes. Then, the hydrofluoric-acidresistant resist was exposed from the both surfaces through a photo-maskhaving a pattern of the external form including portions having thenegative curvature as shown in FIG. 2( a), the exposed hydrofluoric-acidresistant resist was developed using a developer (Na₂CO₃ solution), anda resist pattern was formed where the hydrofluoric-acid resistant resistwas left in areas except etching-target areas on the sheet-shaped glass.

Next, using a mixed acid aqueous solution of hydrofluoric acid andhydrochloric acid as an etchant, the etching-target areas of thesheet-shaped glass were etched from the both main surface sides usingthe resist pattern as a mask, and the glass was cut into the externalform including the portions having the negative curvature as shown inFIG. 2( a). Then, the hydrofluoric-acid resistant resist remaining onthe glass was swollen using an NaOH solution, and was removed from theglass, and rinsing treatment was performed. In this way, obtained wascover glass for a cellular phone of the Example having the external formas shown in FIG. 2( a).

When the cross-sectional shape of the edge face was examined along theexternal form of thus obtained cover glass (Example) using a lightmicroscope, it was confirmed that the cross-sectional shape was theshape as shown in FIG. 4( c) and almost the same over the entireexternal form. Further, surface roughness (arithmetic means roughnessRa) of the main surface of the obtained cover glass was measured usingan atomic force microscope, was 0.2 nm, did not change from the surfacestate immediately after forming by the down-draw method, and had highsmoothness. Furthermore, surface roughness (arithmetic means roughnessRa) of the edge face of the cover glass was measured using the atomicforce microscope, and was 1.2 to 1.3 nm over the entire external form.As the reason why surface roughness of the edge face was thus low, sincethe etching step was adopted without performing mechanical processing inprocessing the external form where the glass substrate was etched withan etchant using a resist pattern formed on the sheet glass as a maskand thereby cut into a desired shape, it is considered that brush marksand/or micro-cracks did not exist that occur in mechanical processing bypolishing grain, grinding grain or the like.

Further, when the presence or absence of micro-crack of the edge face ofthe cover glass was checked using a scanning electron microscope, anymicro-crack was not found.

EXAMPLE 2

The glass cut by etching from which the resist was removed inaforementioned Example 1 was immersed in a treatment bath of mixed acidof 60 percent of potassium nitrate (KNO₃) and 40 percent of sodiumnitrate (NaNO₃) kept at 385° to 405° C. for 4 hours to undergo ionexchange treatment, and chemically strengthened cover glass for acellular phone was prepared where a compressive stress layer of 150 μmwas formed in the glass surface.

When surface roughness of the main surface and edge face was measured asin Example 1, surface roughness of the main surface was 0.3 nm andsurface roughness of the edge face was 1.4 to 1.5 nm. Further, whenchecked whether micro-cracks were present in the edge face, it wasconfirmed that any micro-crack did not exist.

EXAMPLE 3

Cover glass for a cellular phone was prepared as in above-mentionedExample 1 except that the shape of the external form of the cover glasswas a rectangular shape (size: 50 mm×40 mm, thickness: 0.5 mm) as shownin FIG. 5. Surface roughness of the main surface and edge face of thecover glass and micro-cracks were checked as in Example 1, and it wasconfirmed that surface roughness was the same as in Example 1, and thatany micro-crack did not exist.

EXAMPLE 4

As in above-mentioned Example 1 except that a mixed acid aqueoussolution of hydrofluoric acid and nitric acid was used as an etchant,the etching-target areas of the sheet-shaped glass were etched from theboth main surface sides using the resist pattern as a mask, the glasswas cut into the external form including the portions having thenegative curvature as shown in FIG. 2( a), and the cover glass for amobile terminal was prepared. When surface roughness of the main surfaceand edge face of the cover glass and micro-cracks were checked as inExample 1, surface roughness of the main surface was 0.2 nm, surfaceroughness of the edge face was 10 nm, and it was confirmed that anymicro-crack did not exist.

Comparative Example 1

The same aluminosilicate glass as in above-mentioned Example 1 wasformed into plate-shaped glass substrates (sheet-shaped glass) by thedown-draw method. Next, the formed sheet glass was cut into a rectangleslightly larger than finished measurements using a scriber, processedinto a desired shape by grinding the outer edge using a rotary grinderwith diamond grain embedded therein, and processed into the shape asshown in FIG. 6( a).

Next, only the outer edge portion was ground using a diamond grinder,and underwent predetermined chamfering processing. Then, a hundredsheets of plate-shaped glass in the shape as shown in FIG. 6( a) werestacked, and the portion (concave portion 11) having the negativecurvature was formed using a diamond grinder by mechanical processing(FIG. 6( b)).

Subsequently, five sheets of plate-shaped glass in the shape as shown inFIG. 6( b) were stacked, the portion (hole portion 12) having thenegative curvature was formed using a diamond grinder by mechanicalprocessing, and the glass was formed into the shape as shown in FIG. 6(c). Then, finally both main surfaces were polished into mirror surfacesusing ceric oxide, and the sheet thickness was adjusted to 0.5 mm. Inthis way, obtained was cover glass for a cellular phone of ComparativeExample 1 having the shape as shown in FIG. 2( a).

Surface roughness Ra of the main surface and edge face of thus obtainedcover glass (Comparative Example 1) was measured as in the Examples.Surface roughness of the main surface was 0.3 nm, and thus was notdifferent from that in Example 1 so much, but surface roughness of theedge face was 0.2 μm and was a significantly large value. Further, whenthe presence or absence of micro-crack in the edge face of the coverglass was examined, it was confirmed there were many micro-cracks withdepths ranging from dozens to hundreds of micrometers. As the reason whysurface roughness Ra was thus large or many micro-cracks existed, it isconsidered that mechanical processing was adopted in shape processing.

Comparative Example 2

The cover glass of aforementioned Comparative Example 1 was subjected toion exchange treatment under the same conditions as in Example 2, andchemically strengthened cover glass for a mobile terminal was preparedwhere a compressive stress layer was formed in the glass surface. Whensurface roughness of the main surface and edge face of the cover glassand micro-cracks were checked as in Comparative Example 1, it wasconfirmed that surface roughness was the same as in Comparative Example1, and many micro-cracks existed.

Comparative Example 3

Cover glass for a cellular phone was prepared as in Comparative Example2 except that in above-mentioned Comparative Example 1, the shape of theexternal form of the cover glass was made the same shape as shown inExample 3. When surface roughness of the main surface and edge face ofthe cover glass and micro-cracks were checked as in Comparative Example1, it was confirmed that surface roughness was the same as inComparative Example 1, and many micro-cracks existed.

(Evaluation Test of Mechanical Strength of the Cover Glass of Examples 1to 4 and Comparative Examples 1 to 3)

The cover glass was set on a support mount coming into contact with theouter circumferential edge portion in 3 mm in the main surface of thecover glass, and static pressure strength tests were performed whilepressing the center portion of the cover glass from the main surfaceside opposite to the side in contact with the support mount using apressurizing member. Used as the pressurizing material was a materialmade of a stainless alloy having a front end with φ of 5 mm.

As a result, in the cover glass of Examples 1 to 4, the breaking loadwhen the glass was broken exceeded 50 kgf, and the glass had theextremely high mechanical strength. Meanwhile, the cover glass ofComparative Examples 1, 2 and 3 was of 5 kgf, 14 kgf, and 17 kgf, andthus had the significantly low strength. Particularly, the cover glassof Comparative Example 1 was extremely weak in mechanical strength. Whenthe state of cracking of the cover glass of Comparative Example 1 waschecked, it was confirmed that cracking proceeded from micro-cracksexisting in areas including the portions having the negative curvature.

The above-mentioned cover glass for mobile terminals of the invention isapplicable to mobile terminal devices such as cellular phones by beingprovided above a display screen of the apparatus body having the displayscreen. Each of the cover glass of Examples 1 to 4 was provided on adisplay screen of a cellular phone, the cellular phone was subjected toa repetitive drop test, and each cover glass had high mechanicalstrength without any cracking being found.

The present invention is not limited to the above-mentioned Embodiment,and is capable of being carried into practice with modifications thereofas appropriate. For example, the shape of the external form, the numbersof members, sizes, processing procedures and the like in theabove-mentioned Embodiment are examples, and are capable of beingcarried into practice with various modifications thereof within thescope of exhibiting the effects of the invention. Further, the inventionis capable of being carried into practice with modifications thereof asappropriate without departing from the scope of the object of theinvention.

The invention claimed is:
 1. Cover glass for a mobile terminal thatprotects a display screen of the mobile terminal, wherein arithmeticmean roughness Ra is 10 nm or less as surface roughness of an edge face,the cover glass is glass that is chemically strengthened by ion exchangetreatment, and has compressive stress layers in main surfaces and theedge face, and the cover glass is shaped substantially like a rectangle.2. The cover glass for a mobile terminal according to claim 1, wherein amain surface of the cover glass is formed of a molten glass surfaceformed by a down-draw method, and as surface roughness of the mainsurface, arithmetic mean roughness Ra is 0.5 nm or less.
 3. The coverglass for a mobile terminal according to claim 2, wherein the coverglass has a desired shape including a portion having a negativecurvature in part of a contour constituting the cover glass.
 4. Thecover glass for a mobile terminal according to claim 2, wherein a platethickness of the cover glass is 0.5 mm or less.
 5. The cover glass for amobile terminal according to claim 2, wherein the cover glass isaluminosilicate glass containing SiO₂, Al₂O₃ and at least one selectedfrom the group consisting of Li₂O and Na₂O.
 6. The cover glass for amobile terminal according to claim 1, wherein the cover glass has adesired shape including a portion having a negative curvature in part ofa contour constituting the cover glass.
 7. The cover glass for a mobileterminal according to claim 6, wherein a plate thickness of the coverglass is 0.5 mm or less.
 8. The cover glass for a mobile terminalaccording to claim 6, wherein the cover glass is aluminosilicate glasscontaining SiO₂, Al₂O₃ and at least one selected from the groupconsisting of Li₂O and Na₂O.
 9. The cover glass for a mobile terminalaccording to claim 1, wherein a plate thickness of the cover glass is0.5 mm or less.
 10. The cover glass for a mobile terminal according toclaim 9, wherein the cover glass is aluminosilicate glass containingSiO₂, Al₂O₃ and at least one selected from the group consisting of Li₂Oand Na₂O.
 11. The cover glass for a mobile terminal according to claim1, wherein the cover glass is aluminosilicate glass containing SiO₂,Al₂O₃ , and at least one selected from the group consisting of Li₂O andNa₂O.
 12. The cover glass for a mobile terminal according to claim 11,wherein the aluminosilicate glass contains 62 percent to 75 percent byweight of SiO₂, 5 percent to 15 percent by weight of Al₂O₃, 4 percent to10 percent by weight of Li₂O, 4 percent to 12 percent by weight of Na₂O,and 5.5 percent to 15 percent by weight of ZrO₂.
 13. The cover glass fora mobile terminal according to claim 1, wherein the edge face of thecover glass has a protruding center portion and inclined surfaces thatare declined respectively toward both main surface sides from the centerportion, and boundary portions between the inclined surfaces and themain surfaces, respectively, and a boundary portion between the inclinedsurfaces are in a rounder shape.
 14. The cover glass for a mobileterminal according to claim 1, wherein a breaking load in staticpressure strength test exceeds 50 kgf.
 15. The cover glass for a mobileterminal according to claim 1, wherein the cover glass has a holeportion formed through the main surfaces, the hole portion is arrangedat a position shifted to one side in a longitudinal direction of therectangle from a center of each of the main surfaces, and an inner wallface of the hole portion has a surface roughness Ra of 10 nm or less.16. The cover glass for a mobile terminal according to claim 1, whereinthe cover glass has a concave portion formed in an outer circumference,the concave portion is recessed toward an inside of each of the mainsurfaces in a plane, and an inner wall face of the concave portion has asurface roughness Ra of 10 nm or less.
 17. A mobile terminal apparatuscomprising: an apparatus body having a display screen; and a cover glassfor a mobile terminal provided above the display screen, whereinarithmetic mean roughness Ra is 10 nm or less as surface roughness of anedge face, and the cover glass is glass that is chemically strengthenedby ion exchange treatment, and has compressive stress layers in mainsurfaces and the edge face.
 18. The mobile terminal apparatus accordingto claim 17, wherein a main surface of the cover glass is formed of amolten glass surface formed by a down-draw method.
 19. The mobileterminal apparatus according to claim 17, wherein the edge face of thecover glass has a protruding center portion and inclined surfaces thatare declined respectively toward both main surface sides from the centerportion, and boundary portions between the inclined surfaces and themain surfaces, respectively, and a boundary portion between the inclinedsurfaces are in a rounder shape.
 20. The mobile terminal apparatusaccording to claim 17, wherein a breaking load of static pressurestrength test exceeds 50 kgf.